1. Field of the Invention
The present invention relates to an electron display device in which electro luminescence (EL) elements are formed on a substrate, and a method of driving the same. In particular, the present invention relates to an EL display device using semiconductor devices (devices using a semiconductor thin film), and a method of driving the same. The present invention also relates to electronic devices using an EL display device in a display portion.
2. Description of the Related Art
In recent years, EL display devices including EL elements as self light-emitting elements are being actively developed. An EL display device is also called an organic EL display (OELD) or an organic light-emitting diode (OLED).
An EL display device is of a self light-emitting type, unlike a liquid crystal display device. An EL element has a structure in which an EL layer is interposed between a pair of electrodes (anode and cathode), and the EL layer usually has a layered structure. Typically, there is a layered structure “hole transport layer/light-emitting layer/electron transport layer” proposed by Tang of Eastman Kodak. This structure has a very high light-emitting efficiency, and most of the EL display devices that are being studied and developed adopt this structure.
Alternatively, an EL layer may have a structure in which a hole injection layer, a hole transport layer, a light-emitting layer, and an electron transport layer are stacked in this order on an anode or a structure in which a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and electron injection layer are stacked in this order on an anode. A light-emitting layer may be doped with a fluorescent colorant.
In the present specification, all the layers provided between a cathode and an anode are collectively referred to as an “EL layer”. Therefore, the above-mentioned hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, etc. are all included in the EL layer.
A predetermined voltage is applied to an EL layer with the above-mentioned structure through a pair of electrodes, whereby carriers are recombined in a light-emitting layer to emit light. In the present specification, light emission of an EL element is referred to “driving of an EL element”. Furthermore, in the present specification, a light-emitting element composed of an anode, an EL layer, and a cathode is referred to as an “EL element”.
In the present specification, an anode and a cathode of an EL element may be referred to as “both electrodes” of an EL element.
In the present specification, an EL element refers to both an element utilizing light emission (fluorescence) from singlet excitons and an element utilizing light emission (phosphorescence) from triplet excitons.
As a method of driving an EL display device, there is an active matrix system.
FIG. 6 is a block diagram showing an exemplary active matrix type display device. In a pixel portion, source signal lines for receiving a signal from a source signal line driving circuit and gate signal lines for receiving a signal from a gate signal line driving circuit are formed in a matrix. Furthermore, power supply lines are formed in parallel with the source signal lines. In the present specification, the electric potential of the power supply line is referred to as a “power supply potential”.
FIG. 5 shows a structure of a pixel portion of an active matrix type EL display device. Gate signal lines (G1 to Gy) for receiving a selection signal from a gate signal line driving circuit are connected to gate electrodes of switching TFTs 301 of respective pixels. Furthermore, one of a source region and a drain region of the switching TFT 301 of each pixel is connected to a source signal line (S1 to Sx) for receiving a signal from the source signal line driving circuit, and the other is connected to a gate electrode of an EL driving TFT 302 and one electrode of a capacitor 303 of each pixel. The other electrode of the capacitor 303 is connected to a power supply line (V1 to Vx). One of a source region and a drain region of the EL driving TFT 302 of each pixel is connected to the power supply line (V1 to Vx), and the other is connected to an EL element 304 of each pixel.
The EL element 304 includes an anode, a cathode, and an EL layer provided between the anode and the cathode. In the case where the anode of the EL element 304 is connected to the source region or the drain region of the EL driving TFT 302, the anode of the EL element 304 functions as a pixel electrode, and the cathode thereof functions as a counter electrode. On the other hand, in the case where the cathode of the EL element 304 is connected to the source region or the drain region of the EL driving TFT 302, the cathode of the EL element 304 functions as a pixel electrode and the anode thereof functions as a counter electrode.
In the present specification, the electric potential of a counter electrode is referred to as a “counter potential”. A power source for supplying a counter potential to the counter electrode is referred to as a “counter power source”. The potential difference between the electric potential of the pixel electrode and that of the counter electrode is an EL driving voltage, which is applied to the EL layer.
As a gray-scale display method of the above-mentioned EL display device, there are an analog gray-scale system and a time gray-scale system.
First, an analog gray-scale system of an EL display device will be described. FIG. 7 shows a timing chart in the case where the display device in FIG. 5 is driven by the analog gray-scale system. A period, which starts when one gate signal line is selected and finishes when the subsequent gate signal line is selected, is referred to as “one line period (L)”. A period, which starts when one image is selected and finishes when the subsequent image is selected, corresponds to one frame period. In the case of the EL display device in FIG. 5, there are y gate signal lines, so that y line periods (L1 to Ly) are provided in one frame period.
As a resolution is increased, the number of line periods in one frame period is also increased, which makes it necessary to drive a driving circuit at a high frequency.
The power supply lines (V1 to Vx) are kept at a constant potential. The counter potential is also kept constant. The counter potential has a potential difference with respect to the power supply potential to such a degree that an EL element emits light.
In a first line period (L1), a selection signal is supplied to a gate signal line G1 from the gate signal line driving circuit. Then, an analog video signal is successively input to the source signal lines (S1 to Sx). All the switching TFTs 301 connected to the gate signal line G1 are turned on, so that the analog video signals input to the source signal lines S1 to Sx are input to the gate electrodes of the EL driving TFTs 302 through the switching TFTs 301.
The switching TFT 301 is turned on, and the analog video signal input to the pixels becomes a gate voltage of the EL driving TFT 302. At this time, a drain current is determined with respect to a gate voltage in one-to-one correspondence, in accordance with Id-Vg characteristics of the EL driving TFT 302. More specifically, the electric potential of the drain region (EL driving potential in an ON state) is determined so as to correspond to the voltage of the analog video signal input to the gate electrode of the EL driving TFT 302. Then, a predetermined drain current flows through the EL element, and the EL element emits light in a light emission amount corresponding to the current amount.
When the above-mentioned operation is repeated and an input of the analog video signals to the source signal lines (S1 to Sx) is completed, the first line period (L1) is completed. A combination of a period, which finishes when the input of the analog video signals to the source signal lines (S1 to Sx) is completed, and a horizontal retrace period may be defined as one line period. In a second line period (L2), a selection signal is supplied to a gate signal line G2. Then, analog video signals are successively input to the source signal lines (S1 to Sx) in the same way as in the first line period (L1).
When selection signals are supplied to all the gate signal lines (G1 to Gy), all the line periods (L1 to Ly) are completed. When all the line periods (L1 to Ly) are completed, one frame period is completed. In one frame period, all the pixels perform a display, whereby one image is formed. A combination of all the line periods (L1 to Ly) and a vertical retrace period may be defined as one frame period.
As described above, the light emission amount of the EL element is controlled with an analog video signal, and a gray-scale display is performed by controlling the light emission amount. Thus, according to the analog gray-scale system, a gray-scale display is conducted based on variations in a potential of an analog video signal input to a source signal line.
Next, a time gray-scale system will be described.
According to the time gray-scale system, a digital signal is input to a pixel, and a light emission time of an EL element of the pixel is controlled with the digital signal, whereby gray-scale is exhibited.
Herein, the case will be described in which n (n is a natural number of 2 or more) bits of digital signal is input, and a display with 2n gray-scale is conducted.
FIG. 8 shows a timing chart in the case where the display device in FIG. 5 is driven by the time gray-scale system. First, one frame period is divided into n (n is a natural number of 2 or more) sub-frame periods (SF1 to SFn). A period in which all the pixels in a pixel portion display one image is referred to as “one frame period (F)”. A plurality of periods obtained by dividing one frame period correspond to sub-frame periods. As the level of gray-scale is increased, the division number of one frame period is also increased, which makes it necessary to drive a driving circuit at a high frequency.
One sub-frame period is classified into a write period (Ta) and a display period (Ts). The write period refers to a period in which digital signals are input to all the pixels in one sub-frame period. The display period (lighting period) refers to a period in which a light-emitting state or non light-emitting state of an EL element is selected to conduct a display.
The EL driving voltage shown in FIG. 8 represents an EL driving voltage of an EL element with a light-emitting state selected. More specifically, the EL driving voltage of the EL element with a light-emitting state selected becomes 0 volt during a write period. During a display period, the EL driving voltage of the EL element with a light-emitting state selected has a level to such a degree that the EL element emits light.
The counter potential is controlled with an external switch (not shown). The counter potential is kept at the same level as that of the power supply potential during a write period, and has a potential difference with respect to the power source potential to such a degree that an EL element emits light during a display period.
First, a write period and a display period of each sub-frame period will be described in detail by using in FIGS. 5 and 8, and thereafter, the time gray-scale display will be described in detail.
First, a signal is input to a gate signal line G1, and all the switching TFTs 301 connected to the gate signal line G1 are turned on. Then, a digital signal is successively input to the source signal lines (S1 to Sx). The counter potential is kept at the same level as that of the power supply potential of the power supply lines (V1 to Vx). A digital signal has information of “0” or “1”. Digital signals “0” and “1” mean those which have either a Hi voltage or a Lo voltage.
The digital signal input to the source signal line (S1 to Sx) is input to the gate electrode of the EL driving TFT 302 via the switching TFT 301 in an ON state. The digital signal is also input to the capacitor 303 and retained therein.
A signal is input successively to the gate signal lines G2 to Gy, whereby the above-mentioned operation is repeated. All the pixels are supplied with the digital signal, and the digital signal thus input is retained in each pixel. A period up to when all the pixels are supplied with digital signal, is referred to as a “write period”.
When all the pixels are supplied with the digital signal, all the switching TFTs 301 are turned off. Then, the counter potential is changed by an external switch (not shown) connected to the counter electrode, so as to have a potential difference with respect to the power source potential to such a degree that the EL element 304 emits light.
In the case where the digital signal has information of “0”, the EL driving TFT 302 is turned in an OFF state, and the EL element 304 does not emit light. In contrast, in the case where the digital signal has information of “1”, the EL driving TFT 302 is turned in an ON state. Consequently, the pixel electrode of the EL element 304 is kept substantially at the power supply potential, and the EL element 304 emits light. Thus, due to the digital signal, a light-emitting state or a non light-emitting state of the EL element is selected, whereby all the pixels conduct a display at the same time. When all the pixels conduct a display, an image is formed. A period during which pixels conduct a display refers to as a “display period”.
Herein, it is assumed that the lengths of write periods (Ta1 to Tan) of respective n sub-frame periods (SF1 to SFn) are the same, and the display period (Ts) of the respective sub-frame periods (SF1 to SFn) corresponds to Ts1 to Tsn.
For example, the lengths of the display periods Ts1 to Tsn are set so as to be Ts1: Ts2: Ts3: . . . : Ts(n-1): Tsn=20: 2−1: 2−2: . . . : 2−(n-1): 2−(n-1). By combining these display periods, a desired gray-scale display among 2n-level gray-scale can be conducted.
A display period is either one of the periods Ts1 to Tsn. Herein, it is assumed that predetermined pixels are lightened during the period Ts1.
Then, a subsequent write period comes again and all the pixels are supplied with digital signals. Thereafter, a display period comes. At this time, either one of the periods Ts2 to Tsn becomes a display period. Herein, it is assumed that predetermined pixels are lightened during the period Ts2.
Hereinafter, it is assumed that the same operation is repeated with respect to the remaining (n−2) sub-frames, display periods are successively set to be Ts3, Ts4, . . . , Tsn, and predetermined pixels are lightened in each sub-frame.
When n sub-frame periods appear, one frame period is completed. At this time, by adding up the lengths of display periods during which pixels have been lightened, the gray-scale of the pixels is determined. For example, assuming that the brightness in the case where pixels emit light during all the display periods is 100% at n=8, 75% brightness can be exhibited when pixels emit light during the periods Ts1 and Ts2, and 16% brightness can be exhibited when pixels emit light during the periods Ts3, Ts5, and Ts8.
In the present specification, a display period, in which an EL element of a pixel is put in a light-emitting state or a non light-emitting state by a signal of higher order bits among the digital signals input to the display device, is referred to as “a display period of higher order bits”. Furthermore, a display period, in which an EL element of a pixel is put in a light-emitting state or a non light-emitting state by a signal of lower order bits among the digital signals input to the display device, is referred to as “a display period of lower order bits”.
In the case of using a conventional analog gray-scale system, the following problems arise.
The analog gray scale method has the problem that the unevenness of the characteristics of TFTs greatly affects gray scale display. For example, it is assumed that the Id-Vg characteristics of switching TFTs differ between two pixels which represent the same gray scale (the characteristic of either one of the pixels is shifted as a whole to a plus or minus side relative to the characteristic of the other).
In the above-mentioned case, even when the same voltage is applied to the gate electrodes of the respective switching TFTs, drain currents of the respective switching TFTs take different values, and gate voltages with different values are applied to the EL driving TFTs of the respective pixels. In other words, different amounts of currents flow into the EL elements of the respective pixels, and as a result, the amounts of emissions from the EL elements differ from each other and the same gray scale cannot be represented.
Even if equal gate voltages are applied to the EL driving TFTs of the respective pixels, the EL driving TFTs cannot output the same amount of drain current so long as the Id-Vg characteristics of the EL driving TFTs are not even. For this reason, if the Id-Vg characteristics of the switching TFTs slightly differ from each other, the amounts of currents outputted from the EL driving TFTs greatly differ from each other even when equal gate voltages are applied to the EL driving TFTs. As a result, owing to a slight unevenness of the Id-Vg characteristics, the amounts of emissions from the EL elements greatly differ between adjacent pixels even if signals of the same voltage are applied to the EL driving TFTs.
Gray scale display actually becomes far more non-uniform owing to a synergistic effect of the unevenness of the characteristics of the switching TFTs and the unevenness of the characteristics of the EL driving TFTs. Thus, analog gray scale display is extremely sensitive to the unevenness of the characteristics of TFTs. Accordingly, when this EL display device provides gray scale display, there is the problem that the display becomes considerably uneven.
On the other hand, in the case of using a conventional time gray-scale system, the following problems arise.
When the level of gray-scale is increased, the division number of one frame is also increased. Then, in particular, a display period of lower order bits becomes shorter.
In the above-mentioned case, there is a problem that the waveform of a voltage applied to an EL element is corrupted.
In applying a voltage to an EL element during a display period after a write period, voltages of counter electrodes of EL elements of all the pixels are changed at the same time. Therefore, the influence of loads on the EL elements and wirings is very large, so that the waveforms of voltages applied to the EL elements of all of the pixels are corrupted.
In the case where the waveform of a voltage applied to an EL element is corrupted, a predetermined voltage cannot be sufficiently applied to an EL element particularly during a display period of lower order bits that is shortened, which makes it difficult to conduct an exact gray-scale display.
Furthermore, a voltage applied to an EL element in a pixel portion from a power supply line is varied due to the wiring resistance of the power supply line and the like. Therefore, the fluctuation in an applied voltage changes a current to flow through the EL element in the pixel portion, which may cause variations in brightness.
Furthermore, the amount of a current to flow through an EL element is also influenced by a temperature.
Herein, the brightness of an EL element is proportional to a current flowing through the EL element. Therefore, when the current flowing through the EL element is changed, the brightness of the EL element is also changed.
FIG. 4 is a graph showing the changes in the I-V characteristics of an EL element caused by (temperature characteristic). From this graph, it is possible to know the amounts of currents which flow through the EL element with respect to voltages applied across both electrodes of the EL element at certain temperatures. A temperature T1 is higher than a temperature T2, and the temperature T2 is higher than a temperature T3m. As can be seen from FIG. 4, even if the voltage applied across the both electrodes of the EL element in the pixel portion is the same, the current flowing between both electrodes of the EL element becomes larger owing to the temperature characteristic of the EL element as the temperature of the EL element becomes higher. Accordingly, a current to flow through an EL element in a pixel portion is varied due to the environment temperature for an EL display device, and the brightness of the EL element in the pixel portion is changed.
Because of the above, exact gray-scale cannot be exhibited, which is one of the reasons for degrading the reliability of an EL display device.